The present invention, relates generally to drives for workpiece spindles of machine tools.
More particularly, it relates to a drive of the above mentioned general type which includes a drive motor arranged on the free end of the workpiece spindle, wherein the rotor of the drive motor is mounted on the workpiece spindle and the stator is arranged in the motor housing connected with the spindle box of the workpiece spindle.
Drives of the above mentioned type are known in the art and disclosed for example in the Book "Ultraprecision in Manufacturing Engineering", Springer Verlag, 1988, pages 206-207. The workpiece spindle shown in this reference is supported aerostatically in a spindle box. An electric motor is arranged on the end of the workpiece spindle facing away of the workpiece clamping chuck where conventionally the belt pulley of the spindle drive is provided. The rotor is mounted directly on the workpiece spindle.
The German reference DE-PS 2,724,440 proposed a drive which is mounted between the workpiece spindle bearings on the workpiece spindle inside the spindle box. A ring chamber is provided in the hollow workpiece spindle and filled with a heat-conductive medium. Thereby the heat produced by the drive motor is transported to a workpiece spindle region which is surrounded by an air flow chamber. While a thermal shock can be avoided here, a considerable temperature difference is required for withdrawing the produced heat, so that heat expansion of the workpiece spindle does not significantly reduce the accuracy of the working of the workpiece clamped in the machine tool.
The European Patent Document EP-B 94,680 discloses a drive which is also arranged between the bearing points of the workpiece spindle. Here, first of all, the stator is cooled with an air stream and secondly the front stationary bearing is protected from the heat transfer. Between the stationary bearing and the drive motor, a ring chamber for a cooling medium is provided. Here also the problem of heat influence is not solved in a satisfactory manner by the above proposed features, since the workpiece spindle in the region of the motor can be heated unlimitedly. Thereby the whole workpiece spindle can also be heated by the heat transport in metals is very high.
Both last mentioned drives are suitable only for small outputs due to the above mentioned difficulties as well as due to the small space between the bearings. At the same time motors of higher power and thereby higher sizes can be arranged outside the spindle bearings on the ends of the workpiece spindle.